(1) Field of the Invention
The present invention relates to an ion exchange membrane for electrolysis, more specifically to an ion exchange membrane reinforced with a cloth comprising reinforcement threads and sacrificial threads which are used for electrolysis of an alkali metal chloride aqueous solution. Particularly, it relates to a fluorine-containing type ion exchange membrane having an excellent electro-chemical property and an excellent mechanical property which prevents an anode solution from leaking out of a system through a channel after dissolution of sacrificial threads in the membrane.
(2) Description of the Related Art
It is already known in this field that as a solid electrolyte used for a membrane for separation in electrolysis of an alkali metal chloride, laminated membranes of at least two layers containing a perfluorocarbon carboxylic acid layer and a perfluorocarbon sulfonic acid layer are effective. High current efficiency, low electric resistance of a membrane, and easiness of handling are demanded of these ionic membranes, and therefore it is essential for a membrane to have sufficient mechanical strength. However, this perfluorocarbon type film has low tear strength, and does not endure a use for a long period by itself, and therefore tear strength thereof is improved usually by embedding a reinforcing material such as a reinforced cloth, etc. in the film.
However, a general reinforcing material is ionically non-permeable, and therefore when the reinforcing material is embedded in the film, a decrease of an effective electric current area and a raise of electric voltage for electrolysis accompanied thereby are caused on electrolysis. This inclination becomes more remarkable when a structure is made to be dense to raise a reinforcing effect, or a yarn composing the reinforcing material is made to be thick. Moreover, a yarn being made to be thick means that a resinous amount of a membrane to wrap the yarn itself is increased, and it further leads to an increase of electric resistance of the membrane.
To overcome a contradictory relation as above of high mechanical strength of a membrane and low electric resistance of a membrane, various attempts have been conventionally conducted. First of all, an attempt was carried out, namely, a method wherein a structure of a fabric is made to be coarse, and an openness (the total area of windows (apertures among fibers) relative to the total area of the structure of the fabric is represented by a percentage) is enlarged. Generally, in electrolysis of an alkali metal chloride under high electric current density, when the openness is made to be not higher than 70%, an effective electric current area of a membrane becomes short, and not only electric resistance of a membrane is increased, but also a transfer of impurities is locally increased, which causes a decrease of efficiency of electric current. Therefore, usually the openness of not lower than 70% is thought to be necessary.
Then, as an attempt to obtain a reinforced cloth having both high mechanical strength and a large openness, there are proposed a membrane wherein a leno weave cloth highly preventing slippage of stitch is adopted as a cloth, and a yarn of a multi-filament of a perfluoro-polymer having a specified denier is adopted as a yarn used (JP-A-61-7338), and further, a method wherein, after manufacturing the plain weave cloth which has been obtained by mixedly weaving reinforcement threads of a perfluoro-polymer and sacrificial threads soluble in an alkali solution, the sacrificial threads are dissolved, and the only reinforcement threads which remain are inserted between films of a laminate (JP-A-64-55393). However, even by employing these methods as above, an openness of approximately 70% is at most, and when the openness of not lower than 70% is tried to attain, slippage of a stitch at a part of openings of a cloth is caused, which makes difficult a production of a cloth, and insertion thereof between films of a laminate.
Furthermore, there is proposed a cloth using, instead of sacrificial threads, a yarn having a raised apparent specific gravity by improving a commercially available yarn of polytetrafluoroethylene (PTFE) having porosity. However, employment of only the reinforcement threads is limitative to raising of openness (JP-A-3-217427).
Thus, a method is proposed wherein the plain weave reinforced cloth which has been obtained by mixedly weaving a reinforce yarn of a perfluoro-polymer and a sacrifice yarn which is soluble on use in an electrolysis cell, or is soluble by a chemical treatment such as with an acid or an alkali solution, is inserted between films of a laminate, and then the sacrificial threads in the cloth are dissolved by the chemical treatment as described above (JP-A-1-308435 and JP-A-63-113029). By mixedly weaving with sacrificial threads, even when the openness at a portion of reinforcement threads is high, this cloth maintains favorable prevention from slippage of a stitch. Further, since sacrificial threads are dissolved while remaining in a membrane, at the part where the sacrificial threads originally occupied, a void channel (hereinafter referred to as xe2x80x9ca channel after dissolution of sacrificial threadsxe2x80x9d) is produced in the membrane. Moreover, by making the position of the cloth in the membrane close to the side of the membrane which is in contact with an anode solution, a minute cleft (hereinafter referred to as xe2x80x9cpenetrating channelxe2x80x9d) is caused on the surface of the membrane, and by conducting the anode solution thorough the penetrating channel to a continuous tubular path which has been formed by a channel after dissolution of sacrificial threads at an inner part of the membrane, the anode solution can be filled to the part where an ionic transfer has been prevented by the reinforcement threads and the layer where the tubular path resides. Resultantly, electric resistance of a membrane can be reduced.
However, there is such a problem that the tubular path is in connection with the whole of the cloth, namely with the whole of the membrane, on use in an electrolysis cell, a portion of the anode solution is apt to bleed out of a flange which fixes a membrane to the electrolysis cell, and thus a leak of the anode solution from the circumference of the membrane is caused. This leak of the anode solution out of the vessel accelerates corrosion of an electrolysis cell and deterioration of a gasket, and, in the worst case, a short circuit is caused owing to a precipitation of a salt, and electrolysis sometimes is forced to be terminated. Particularly, pressure on the surface of the flange in the longitudinal direction of the electrolysis cell sometimes is not uniform, and leak especially from the lower part of the electrolysis cell can be caused. Therefore, when the electrolysis cell is equipped with a membrane, a channel after dissolution at the flange part is clogged, for instance, by applying a pasty silicone sealant or a fluorine type grease to the gasket. However, depending upon the shape of an electrolysis cell, the coating takes much time and labor, and when thickness of the coating is not uniform, the sealant or the grease may protrude to an electric current conducting portion and an electrolysis cell, which are a problem.
The task of the present invention is to provide a cation exchange membrane using a plain weave reinforced cloth, and a process of a production thereof, wherein the membrane has a channel(s) after dissolution which is(are) formed at a part(s) where sacrificial threads have been dissolved, has a continuous tubular path(s) formed thereby, and on use of an electrolysis cell, no leak of an anode solution can be seen out of the membrane through the channel(s) after dissolution and the tubular path(s).
The present inventors have intensively studied to solve the problems as stated above, and as a result, they have found that, when the shape of a cross section of a channel after dissolution which is formed at a part where sacrificial threads have been dissolved, is made to be flat in a direction of a plane of the membrane, remarkable effect can be exhibited to solve the problems as stated above. Thus, the present invention has been accomplished.
Namely, the present invention relates to:
(1) A cation exchange membrane wherein a plain weave reinforced cloth is embedded in a fluorinated polymer having a sulfonic acid group and/or a carboxylic acid group, characterized by possessing therein a tubular path which is formed in a direction of a warp and a weft of the plain weave reinforced cloth and has a cross section flat to a direction of thickness of the membrane;
(2) The cation exchange membrane according to the above-described (1), wherein the tubular path has been formed by dissolution of sacrificial threads, and the sacrificial threads are of 20 to 50 denier and consists of a multi-filament composed of 4 to 8 mono-filaments and having a circular cross section;
(3) The cation exchange membrane according to the above-described (1), wherein a thickness of the plain weave reinforced cloth is from 30 to 80 xcexcm;
(4) The cation exchange membrane according to the above-described (1), wherein reinforcement threads of the plain weave reinforced cloth are stretched yarn of a polytetrafluoroethylene having a circular cross section and having a specific gravity of 2.0 to 2.3;
(5) A process for producing a cation exchange membrane having therein a tubular path, characterized in that a plain weave reinforced cloth which has been mixedly weaved with reinforcement threads and sacrificial threads, and wherein the sacrificial threads are flat parallel to a plane of the cloth, or mono-filaments composing the sacrificial threads reside in a row without overlapping one another in the direction of thickness of the cloth, is embedded in a fluorinated polymer having a sulfonic acid group and/or a carboxylic acid group, to form a membrane, and thereafter the membrane is hydrolyzed and at the same time the sacrificial threads are dissolved with an acid or an alkali;
(6) The process for producing a cation exchange membrane according to the above-described (5), wherein a number of twisting of the sacrificial threads of the warp is not higher than 250 times/m, and a number of twisting of the sacrificial threads of the weft is not higher than 200 times/m;
(7) The process for producing a cation exchange membrane according to the above-described (5), wherein the sacrificial threads are composed of a multi-filament of polyethylene terephthalate;
(8) The process for producing a cation exchange membrane according to the above-described (5), wherein the sacrificial threads are of 20 to 50 denier and consist of a multi-filament composed of 4 to 8 mono-filaments and having a circular cross section;
(9) The process for producing a cation exchange membrane according to the above-described (5), wherein a shrinkage ratio of the sacrificial threads of the warp in boiling water is not lower than 6%, and a shrinkage ratio of the sacrificial threads of the weft in boiling water is not higher than 3%;
(10) The process for producing a cation exchange membrane according to the above-described (5), wherein a number of twisting of the sacrificial threads of the weft is higher than 0 times/m and not higher than 100 times/m;
(11) The process for producing a cation exchange membrane according to the above-described (5), wherein a thickness of the plain weave reinforced cloth is 30 to 80 xcexcm; and
(12) A precursor of a cation exchange membrane wherein a plain weave reinforced cloth which, by employing sacrificial threads having a number of twisting of not higher than 250 times/m as a warp, and sacrificial threads having a number of twisting of not higher than 200 times/m as a weft, has been prepared by mixedly weaving them with reinforcement threads, is embedded in a fluorinated polymer having a sulfonic acid group and/or a carboxylic acid group, and the sacrificial threads are flat parallel to a plane of the membrane, or mono-filaments composing the sacrificial threads reside in a row without overlapping one another in a direction of thickness of the membrane and are in a continuous form in the direction of each of to the warp and the weft of the cloth.
The reinforced cloth according to the present invention can render the shape of a cross section of a channel after dissolution of sacrificial threads flat in a direction of a plane of the membrane, the channel after dissolution can be easily crushed at a portion of a cell flange of an electrolysis cell, the effect of preventing leak of an anode solution out of the cell flange can be exhibited. Thereby, the conventional applying of a silicone sealant or a fluorine type grease becomes unnecessary, and easy setting of the membrane to an electrolysis cell becomes possible.